Method of determining weathering corrections in seismic operations

Abstract

In a seismic recording and processing system in which a line of sources and a separate line of receivers are provided and each of the source operations from each of the source points is recorded on all receiver points. A method is described for determining the differential static correction between each adjacent pair of source points in terms of the differences in travel times from each pair of source points, to all pairs of receiver points. This is provided even though the lines of source points and receiver points may be parallel to or at angles to each other, and the spacings of source points and receiver points in their separate lines may be the same or different.

What is claimed is: 
     
       1. In a seismic system having at least one line of spacial source points (SPs) A,B,C, the position of each SP known; and having at least one different line of spaced receiver points (RPs), D,E,F,G, the positions of each RP known; to the lines not colinear. the method of determining the differential static time correction by transmission through the weathered layer, from the records from adjacent closely spaced SPs A and B, comprising the steps of;   (a) recording a first record from a first SP, A to a plurality of closely spaced RPs D,E,F,G;   (b) recording a second record from a second SP, B to a plurality of closely spaced RPs D,E,F,G;   (c) determining the horizontal distances from SP A to each of the RPs D,E,F,G,; and from SP B to each of the RPs D,E,F,G,;   (d) selecting a first pair of traces from said first and second records, in which the path lengths from adjacent SPs such as A,B, to a selected pair of RPs such as D,E, differ in length by no more than a selected small dimension dE, for which the differential travel time in the subsurface dt, would be equal to dE/V, where V is the horizontal subweathering velocity;   (e) determining the differential travel time between the two paths from A and B to D and E, applying the time correction dt algebraically to compensate for different lengths of paths, and determining the first timing difference T1 in travel times through the weathered layer at source points A and B and RPs D and E;   (f) repeating steps (d) and (e) for a second pair of traces from the same pair of SPs A and B to a second pair of RPs, such as E and F, adjacent to said first pair; applying the proper time correction dt and determining the second time difference T2 in travel times through the weathered layer; and   (g) determining the average value TA of the time differences T1 and T2 of said plurality of pairs of paths.   
     
     
       2. The method as in claim 1 including the additional steps of repeating steps (d)(e)(f) and (g) for all pairs of traces from SPs A and B and determining the average value TA of all of said plurality of time differences. 
     
     
       3. The method as in claim 1 in which said first step of determining the differential travel time between the two paths comprises correlating selected equal initial short portions of said first and second traces, whereby only refracted seismic waves at the base of weathering are correlated. 
     
     
       4. The method as in claim 3 in which said selected portions of said traces include the portion from the first breaks to a selected time interval after said first breaks. 
     
     
       5. The method as in claim 4 in which said selected time interval is less than 100 milliseconds. 
     
     
       6. The method as in claim 4 in which said selected time interval is less than 250 ms. 
     
     
       7. The method as in claim 1 in which at least one of said lines of RPs and SPs is curved. 
     
     
       8. The method as in claim 1 including at least one line of RPs and at least one parallel line of SPs spaced from said line of RPs. 
     
     
       9. The method as in claim 8 including at least two parallel spaced lines of RPs and at least one parallel line of SPs spaced from said lines of RPs. 
     
     
       10. The method as in claim 9 in which said line of SPs is in the space between said at least two lines of RPs. 
     
     
       11. The method as in claim 9 in which said line of SPs is outside of the space between said at least two lines of RPs. 
     
     
       12. The method as in claim 9 including also an additional plurality of spaced RPs alongside of said line of SPs, and spaced therefrom. 
     
     
       13. The method as in claim 1 including at least one line of RPs and one line of SPs in angular relation such that said two lines intersect each other. 
     
     
       14. The method as in claim 1 including at least two parallel lines of SPs and at least two parallel lines of RPs, said at least two pairs of lines crossing each other at a selected angle. 
     
     
       15. The method as in claim 1 in which the source is impulsive. 
     
     
       16. The method as in claim 1 in which the source is vibratory. 
     
     
       17. The method as in claim 1 in which the source is a surface source. 
     
     
       18. The method as in claim 1 in which the spacings between adjacent RPs in the line of RPs are the same. 
     
     
       19. The method as in claim 1 in which the spacings between the SPs in the line of SPs are the same. 
     
     
       20. The method as in claim 1 in which the spacings between the SPs is the same as the spacings between the RPs. 
     
     
       21. The method as in claim 1 including the step of discarding any pairs of paths for which the differential distance is greater than the selected value dE. 
     
     
       22. In a seismic system having at least one line of spaced source points (SPs) such as A,B,C,D the position of each SP known; and having at least one different line of spaced receiver points (RPs) such as E,F,G, the position of each RP known; the two lines not colinear; the method of determining the differential static correction by transmission through the weathered layer between two closely spaced RPs such as E,F, from the records taken from a plurality of closely spaced SPs, comprising the steps of:   (a) recording a first record from a first SP A to at least two closely spaced RPs E and F, and a second record from a second SP B to at least E and F, and a third record from a third SP C to at least E and F;   (b) determining the horizontal distances from each RP E and F to each of the plurality of SPs A,B,C,D;   (c) selecting a first pair of traces from said first and second records in which the path lengths from adjacent SPs such as A,B, to a selected pair of RPs such as E,F differ in length by no more than a selected small dimension dE, for which the differential travel time in the subweathering would be equal to dE/V where V is the horizontal subweathering velocity;   (d) determining the differential travel time between the two paths from A,B to E,F; and applying the time correction dt algebraically to compensate for different lengths of paths; and determining the first timing difference T1 in travel times through the weathered layer at receiver points E and F;   (e) repeating steps (c) and (d) for a second pair of traces from said second and third records from said SPs B and C to at lesat RPs E and F, applying the proper time correction dt and determining the second time difference T2 in travel times through the weathered layer; and   (f) determining the average value TA of the time differences T1 and T2 of said plurality of pairs of paths.